It is known in the field of vehicle seats to interpose a heater element in sheet form between a cushion pad and a skin member forming a seat cushion or a seat back for the purpose of heating a prescribed part of the seat. The heater element typically comprises a substrate in sheet form, and a resistive heater wire printed on the substrate. During the assembly process of the vehicle seat, the heater element is typically placed on the surface of the cushion pad which is formed with a tuck-in groove, and a part of the heater element crossing the tuck-in groove is tucked into the tuck-in groove along with the skin member, See JP2014-36730A, for instance.
When the cushion pad is compressed by an occupant seating on the upper surface of the seat cushion, the cushion pad is compressed, and the upper part of the cushion pad adjoining the tuck-in groove is displaced downward. As a result, the heater wire in the part of the heater element crossing the tuck-in groove is bent. An excessive bending of the heater wire is not desirable in terms of the durability of the heater wire. To minimize the bending of the heater element in the tuck-in groove, the heater wire may extend across the tuck-in groove in a wavy pattern or a zigzag pattern. However, it has been found that the corners of the wavy pattern or the zigzag pattern or portions where the extending direction of the heater wire changes are subjected to concentrated stress so that the durability of the heater wire may be impaired after repeated compression and decompression of the cushion pad.